Technical
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Since each of these systems absorbs heat in one area and rejects it in another , we would find at least two heat exchangers of some kind in any of these systems .
Heat transfer was “ extended ” with the use / addition of fins . The transfer rate ( W ) in DX applications could be defined as follows . ( Stoeker , 1976 , p163 ): q = Duty Watts U = Overall heat transfer coefficient
W / m ² A = Surface Area m ² ( mean surface area ) tf = Fluid temperature / Air temperature ° C tr = Refrigerant temperature ° C
The overall heat transfer coefficient , being a product of the materials used and their respective wall thicknesses . With reduction in the difference in temperature between tf and tr the surface area ( A ) would need to be bigger in order to achieve the same potential heat transfer .
There is a wide variety of HE types on offer with new developments contributing to the options to consider in specific applications . A very commonly used type of HEs would be the fin and tube type . There are various reasons for this trend . One of the most obvious would be the fact that volumes of air are drawn across the HE , in most cases , to offer the cooling effect or to reject the heat from a functioning system ( in the case of a air cooled condenser ).
The air across the fin and tube coil is being cooled or provide cooling .
“ Why make condensers air cooled ?” This is a question I have received a number of times and I believe the best answer is simply that the ambient air volumes you have to make use of are in abundant supply and “ free of charge ”. In the case of an air-cooled condenser the ambient air drawn into the unit would gain some of the heat energy on the inside of the coil tubes as it comes into contact with the coil tubes and fins .
TYPES OF FIN AND TUBE HEAT EXCHANGERS COMMONLY USED Due to the relative ease of manufacturing fin and tube designs , these are used in a variety of HVACandR applications . Not limited to these of course , but these would be the most commonly found in operation .
• Air cooled condensers – DX systems An air-cooled condenser is commonly used as part of any direct expansion ( DX ) type system to reject heat energy from the system . The heat energy resulting from the compressor ’ s
Science Direct www . assemblymag . com Marius La Grange
function also known as the input power ( IP ), plus heat energy required by the system to absorb ( evaporative duty ) is known as the system ’ s total heat rejection ( THR ). The IP being a by-product of a DX system .
Ambient air being the fluid that the heat energy is transferred towards , is drawn across the coil surfaces ( in most cases ). The air is forced across the coils surface area and the greater the mass flow of air , the greater the potential heat rejection .
Cooling loads vary in many systems so the required THR varies as well . This could be done making use of multiple fans and staging them or by using a variable speed fan arrangement . The greater the difference in temperature ( ΔT ) between the targeted condensing temperature
Fin and tube coil ( dry cooler application ).
Inner groove copper tube options .
An aluminium fin plate with spacing collar .
www . hvacronline . co . za RACA Journal I April 2021 49